Circuit substrate

ABSTRACT

A circuit substrate which has a ceramic substrate and an Al circuit comprising Al or an Al alloy bonded to said ceramic substrate via a layer comprising Al and Cu.

[0001] The present invention relates to a highly reliable circuitsubstrate useful for e.g. power modules.

[0002] Conventionally, for semiconductor devices useful for e.g. powermodules, circuit substrates having a ceramic substrate of e.g. alumina,beryllia, silicon nitride or aluminum nitride, and a circuit and aheat-radiating plate made of e.g. Cu, Al or an alloy of such a metalcomponent, formed respectively on the front and on the back side of theceramic substrate, have been developed (U.S. Pat. No. 5,354,415) andused practically. These circuit substrates have such a merit that highinsulating properties will be obtained stably as compared with resinsubstrates or composite substrates of a resin substrate and a metalsubstrate.

[0003] Methods for bonding the circuit and the heat-radiating plate tothe ceramic substrate are classified roughly into brazing employing abonding material (i.e. brazing material) and a method of not employing abonding material. As a representative method of the latter, DBC methodof bonding alumina to a tough pitch copper plate by utilizing the Cu—Oeutectic point has been known.

[0004] However, in the case where the circuit is made of Cu, the thermalstress resulting from difference in thermal expansion between thecircuit and the ceramic substrate or the solder is inevitable, and thuscracks tend to form on the ceramic substrate or the solder due torepeated heat history, and no adequately high reliability will beobtained. On the other hand, when Al is selected for the material forthe circuit, although it is somewhat poor in thermal conductivity andelectrical conductivity as compared with Cu, the Al circuit will easilyundergo plastic deformation even if thermal stress is applied thereto,whereby the stress to be applied to the ceramic substrate or the solderwill be moderated, and the reliability will significantly improve.

[0005] However, the Al circuit has such a problem that it is expensive.To form an Al circuit, there are following methods: (1) a melting methodin which a molten aluminum is contacted with a ceramic substrate,followed by cooling to produce a bonded product, and the thickness ofthe Al plate is adjusted by machine grinding, followed by etching(JP-A-7-193358, JP-A-7-27262) and (2) a method of brazing an Al foil oran Al alloy foil, followed by etching (JP-A-3-125463). Both the methodswill cost from about twice to about five times as much as the case offorming a Cu circuit, and thus there is a little possibility that thesemethods are used widely, except for a special purpose.

[0006] Not to speak of the melting method wherein the productionefficiency is poor, the major cause why the Al circuit by brazing ismore costly than a Cu circuit, is that the bonding is carried out undersevere conditions. Namely, the melt temperature of Al (660° C.) and thebonding temperature (from about 630 to about 650° C. in the case of anAl—Si type which is the commonest bonding material) are close, wherebyAl is likely to melt locally to cause soldering defects (moth-eatenphenomenon formed on the Al circuit), and thus considerable skill andlabor are required to produce the Al circuit while preventing suchdefects.

[0007] Under these circumstances, the present inventors have found thatan Al circuit can be easily formed on a ceramic substrate by bondingunder specific conditions by using, as a bonding material, a low-pricedAl—Cu type alloy which has attracted no attention, and they have furtherconducted extensive studies to accomplish the present invention.

[0008] It is an object of the present invention to provide a circuitsubstrate which has a ceramic substrate and an Al or Al alloy circuitformed on the ceramic substrate, at a low cost while keeping its highreliability. Particularly, it is to provide a highly reliable circuitsubstrate wherein not only cracks on a solder or the ceramic substratebut also peeling of a bonding wire or plating is significantlyprevented.

[0009] Another object of the present invention is to produce such ahighly reliable circuit substrate easily.

[0010] Namely, the present invention resides in a circuit substrate anda process for producing the circuit substrate, which have the followingessential features.

[0011] 1. A circuit substrate which has a ceramic substrate and an Alcircuit comprising Al or an Al alloy bonded to said ceramic substratevia a layer comprising Al and Cu.

[0012] 2. A circuit substrate which has a ceramic substrate and an Alcircuit comprising Al or an Al alloy bonded to said ceramic substrate byusing, as a bonding material, an Al—Cu type alloy or a mixturecontaining Al and Cu.

[0013] 3. The circuit substrate according to Item 2, wherein the bondingmaterial is an Al—Cu type alloy foil.

[0014] 4. The circuit substrate according to Item 1, 2 or 3, wherein theceramic substrate is an aluminum nitride substrate or a silicon nitridesubstrate.

[0015] 5. The circuit substrate according to any one of Items 1 to 4,wherein the ceramic substrate is an aluminum nitride substrate having athermal conductivity of at least 130 W/mK and having such a X-raydiffraction peak intensity ratio that 2≦Y₂O₃.Al₂O₃×100/AlN≦17 and2Y₂O₃.Al₂O₃×100/AlN≦2 on the surface.

[0016] 6. The circuit substrate according to any one of Items 1 to 5,wherein the Al circuit comprising Al or an Al alloy is formed by usingAl having a purity of at least 99.85 wt %.

[0017] 7. The circuit substrate according to any one of Items 1 to 6,wherein the Al circuit comprising Al or an Al alloy is formed by using arolled Al having a purity of at least 99.99 wt %.

[0018] 8. The circuit substrate according to Item 1, wherein theproportion of Cu in the layer comprising Al and Cu is from 1 to 6 wt %.

[0019] 9. The circuit substrate according to Item 2 or 3, wherein thebonding material comprises at least 86 wt % of Al, from 1 to 6 wt % ofCu and at most 3 wt % of Mg (not including 0).

[0020] 10. The circuit substrate according to any one of Items 1 to 9,wherein the Al circuit comprising Al or an Al alloy has a thickness ofat least 100 μm and a Vickers hardness of at most 15 kgf/mm².

[0021] 11. The circuit substrate according to any one of Items 1 to 10,which has a heat-radiating plate comprising Al or an Al alloy formed onthe ceramic substrate on the side (back side) opposite to the side onwhich the Al circuit is formed.

[0022] 12. The circuit substrate according to Item 11, wherein thevolume ratio of the Al circuit to the heat-radiating plate is from 0.80to 1.2.

[0023] 13. The circuit substrate according to Item 11 or 12, wherein theAl circuit has a Vickers hardness of at most 16 kgf/mm², and theheat-radiating plate has a Vickers hardness of from 19 to 30 kgf/mm².

[0024] 14. A process for producing a circuit substrate, which comprisesdisposing an Al or Al alloy plate, pattern or both on a ceramicsubstrate by means of, as a bonding material, an Al—Cu type alloy or amixture containing Al and Cu, and heating the resulting assembly at atemperature of from 540 to 640° C. while applying a pressure of from 1to 100 kgf/cm² thereto in a direction perpendicular to the ceramicsubstrate to bond the Al or Al alloy plate, pattern or both to theceramic substrate, followed by etching as the case requires.

[0025] 15. The process for producing a circuit substrate according toItem 14, wherein the bonding material is an Al—Cu type alloy foil.

[0026] 16. The process for producing a circuit substrate according toItem 15, wherein the bonding material is an Al—Cu type alloy foil havinga thickness of from 15 to 35 μm, the Al or Al alloy plate, pattern orboth, having a thickness of at least 100 μm, is disposed on either sideof the ceramic substrate by means of said bonding material, and theresulting assembly is held under heating at a temperature of at least590° C. for at least 20 minutes while applying a pressure of from 8 to50 kgf/cm² thereto in a direction perpendicular to the ceramicsubstrate.

[0027] 17. The process for producing a circuit substrate according toItem 15 or 16, wherein the Al—Cu type alloy foil is an Al—Cu—Mg typealloy foil comprising at least 86 wt % of Al, from 1 to 6 wt % of Cu andat most 3 wt % of Mg (not including 0).

[0028] Now, the present invention will be described in detail withreference to the preferred embodiments.

[0029] In the accompanying drawing:

[0030]FIG. 1 is a diagram illustrating the process for producing acircuit substrate composite.

[0031] The present invention is greatly characterized by that an Alcircuit or an Al alloy circuit (hereinafter both will be referred to asAl circuit) is formed on at least one side of a ceramic substrate via alayer comprising Al and Cu. In other words, an Al circuit is bonded toat least one side of a ceramic substrate by using, as a bondingmaterial, an Al—Cu type alloy or a mixture containing Al and Cu. Thecircuit substrate of the present invention may have such a structurethat a heat-radiating plate is formed on the ceramic substrate on theside (back side) opposite to the side on which the Al circuit is formed.

[0032] Heretofore, as the bonding material for the Al circuit, an Al—Sitype alloy has been most well known, and some studies have been made one.g. an Al—Si—Mg type, an Al—Ge type and an Al—Si—Ge type. However, noprior art has been found wherein an Al—Cu type alloy is used as thebonding material. It is considered that the Al—Cu type alloy hasattracted no attention, since it is relatively hard and fragile, whichprepossessed users against it to be disadvantageous to plasticdeformation which will release thermal stress of the circuit substrate.

[0033] However, from the viewpoint of easiness in bonding of the Alcircuit, the Al—Cu type alloy is absolutely favorable as compared withan Al—Si type, an Al—Ge type or a type having Mg added thereto, since Cuis likely to diffuse uniformly in Al as compared with Si or Ge, and thusno local melting nor spewing of superfluous bonding material is likelyto result, and accordingly the bonding can be carried out stably in arelatively short time.

[0034] Further, as an alloy of AA symbol series 2000, it is widely usedas a high-strength Al alloy or as a heat-resistant Al alloy, and it iseasily formed into a foil, such being favorable in view of cost also.

[0035] In the circuit substrate of the present invention, an Al circuitis bonded to a ceramic substrate via a layer comprising Al and Cu. Sucha layer is constituted by a layer which contains at least the twocomponents of Al and Cu, and may further contain a third component. Asthe third component, e.g. Mg, Zn, In, Mn, Cr, Ti or Bi may, for example,be contained in a total amount of at most about 5 wt %. Among them, Mgis preferred.

[0036] The layer comprising Al and Cu is located on the ceramicsubstrate preferably within 100 μm from the surface of the ceramicsubstrate. Between the Al circuit and the layer comprising Al and Cu, athird layer may or may not be present.

[0037] The layer comprising Al and Cu is formed in such a manner that ona ceramic substrate, an Al or Al alloy plate, pattern or both isdisposed by means of an Al—Cu type bonding material, followed by bondingunder heating while applying pressure thereto.

[0038] As the bonding material, an Al—Cu type alloy or a mixturecontaining Al and Cu is used. Among them, preferred is an Al—Cu typealloy foil, particularly an alloy foil having a thickness of from{fraction (1/10)} to {fraction (1/50)} of the thickness of the Alcircuit. If the thickness is less than {fraction (1/50)}, no adequatebonding will be carried out, and if it exceeds {fraction (1/10)}, the Alcircuit tends to be hard, such being unfavorable to the heat history ofthe circuit substrate. The thickness is particularly preferably at most100 μm, and from {fraction (1/12)} to {fraction (1/40)} of the thicknessof the Al circuit. As an Al circuit having a thickness of from 0.4 to0.6 mm is usually used, the thickness of the bonding material is from 10to 50 μm, particularly from about 15 to about 30 μm.

[0039] The bonding material comprises preferably at least 86 wt % of Al,from 1 to 6 wt % of Cu, and at most 3 wt %, particularly from 0.2 to 2.0wt %, of Mg (not including 0).

[0040] To obtain a further highly reliable circuit substrate, the Al—Cutype alloy comprises preferably at least 86 wt % of Al, from 1 to 6 wt %of Cu and at most 3 wt % of Mg (not including 0). If the content of Cuis less than 1 wt %, the bonding temperature tends to be high and closeto the melting point of Al, and if it exceeds 6 wt %, Cu tends todiffuse in the Al circuit after the bonding, such being unfavorable tothe heat history of the circuit substrate. The content of Cu ispreferably from 1.5 to 5 wt %.

[0041] The content of Cu in the layer comprising Al and Cu is determinedsubstantially by the content of Cu in the bonding material to be usedfor the bonding.

[0042] When Mg is added, the characteristics of the Al—Cu type alloywill be exploited, and the adhesion of the Al circuit to the ceramicsubstrate will be improved.

[0043] The mechanism how the effects by Mg addition appear is notclearly understood in detail. However, it is estimated that Mg reactswith an oxide layer on the surface of Al to form MgO and to remove theoxide layer, and MgN₂ is formed on the surface of the ceramic substrate,whereby the wettability will improve.

[0044] If the content of Mg exceeds 3 wt %, Mg will evaporate in largequantities during the bonding operation so that e.g. the Al circuit maybe broken, or Mg will diffuse in large quantities in the Al circuit sothat Al may undergo significant curing. If the content of Mg is too low,the effect to improve the adhesion tends to be small, and accordingly,the content of Mg is preferably from 0.2 to 2.0 wt %.

[0045] Further, a fourth content such as Zn, In, Mn, Cr, Ti, Bi, B or Femay be incorporated in a total amount of at most about 5 wt %. By usinga bonding material having such a composition, a circuit substrate willbe provided more stably at a lower cost.

[0046] Specific examples (commercially available alloys) of the bondingmaterial include an Al—Cu alloy having a Cu content of from 1 to 6 wt %,2018 alloy containing about 4 wt % of Cu and about 0.5 wt % of Mg, 2017alloy containing about 0.5 wt % of Mn, and JIS alloys 2001, 2003, 2005,2007, 2011, 2014, 2024, 2025, 2030, 2034, 2036, 2048, 2090, 2117, 2124,2218, 2224, 2324 and 7050.

[0047] Now, the Al circuit will be explained.

[0048] As the material for the Al circuit, in addition to 1000 seriespure Al, 4000 series Al—Si type alloys with which the bonding is easilycarried out, and 6000 series Al—Mg—Si type alloys may be used. Amongthem, preferred is a high purity Al (purity: at least 99.85 wt %) havinga low breakdown proof stress. Such an Al plate is commercially availableas 1085 or 1N85 material. Further, an Al having a purity of 99.9 wt %(3N), an Al having a purity of 99.99 wt % (4N) and an Al having a purityof 99.999 wt % (5N) may be used since they are not so expensive.

[0049] The Al circuit may be a simple substance or a laminate such as aclad of at least two kinds. Examples of the laminate include Al—Ni,Al—Ni—Cu, Al—Mo, Al—W and Al—Cu. They may be optionally selectedaccording to the purpose of use and the bonding method. However, it ispreferred to use a rolled plate of an Al simple substance having apurity of at least 99.99 wt %, particularly a rolled plate having areduction ratio of at least 10%. The reason why a rolled Al plate ispreferred is that uniform rolling will be carried out repeatedly by aroll, whereby uniform plastic deformation is likely to result, ascompared with the above melting aluminum method.

[0050] The thickness of the Al circuit is usually from 0.3 to 0.5 mm. Ifit significantly departs from this range, the above preferred relationto the thickness of the bonding material is less likely to bemaintained. For example, when the thickness of the Al circuit is 3 mm,the thickness of an alloy foil of 200 μm which is {fraction (1/15)} ofthe thickness of the Al circuit, is not appropriate, and a hard layerunfavorable to the heat history tends to be formed.

[0051] Further, of the Al circuit, the thickness is preferably at least100 μm and the Vickers hardness is preferably at most 15 kgf/mm², so asto optimize the hardness of the Al circuit to achieve relatively uniformplastic deformation, to prevent peeling of a bonding wire and plating,and to significantly reduce damages such as solder cracks.

[0052] The thickness of at least 100 μm of the Al circuit is a necessarycondition to obtain a diffusion distance of the bonding material ofabout several tens μm and to make the Vickers hardness of the Al circuitat most 15 kgf/mm². If the Vickers hardness exceeds 15 kgf/mm², theplastic deformation tends to be non-uniform when the Al circuit issubjected to thermal stress, whereby partial deformation tends to besignificant, and plating or a bonding wire tends to peel, or damagessuch as solder cracks tend to be significant. The lower limit of theVickers hardness is not particularly limited, and the smaller, thebetter. However, it is preferably from 10 to 14 kgf/mm², since the Alcircuit tends to be damaged if it is too soft.

[0053] With respect to a circuit substrate having an Al circuit formedthereon, in order to increase the reliability, proposes have beenconventionally made such as a plating composition (JP-A-8-260187),modification of the surface of the ceramic substrate (JP-A-8-260186) anda particle size definition of the Al plate (JP-A-8-156330). However,they are not satisfactory solvents. On the other hand, according to thepresent invention, the problems can easily be overcome by optimizing thethickness and the Vickers hardness of the Al circuit.

[0054] With respect to a circuit substrate having such a structure thata heat-radiating plate is formed on the opposite side (back side) of theceramic substrate, it is preferred to make the volume ratio of the Alcircuit to the heat-radiating plate (the volume of the circuit/thevolume of the heat-radiating plate) in the vicinity of 1, in order tominimize warps and swells due to thermal stress, and to adequatelyprevent damages such as solder cracks and peeling of a bonding wire andplating. Said ratio is preferably from 0.80 to 1.2, particularlypreferably from 0.85 to 1.15, furthermore preferably from 0.90 to 1.1.In such a case, the thickness of the heat-radiating plate is preferablyequal to or less than the thickness of the circuit.

[0055] The volume of the Al circuit and the volume of the heat-radiatingplate are calculated from formulae (circuit area)×(circuit thickness)and (heat-radiating plate area)×(heat-radiating plate thickness),respectively.

[0056] Also in the case where the volume ratio is as mentioned above,the Vickers hardness of the Al circuit is particularly preferably atmost 16 kgf/mm², and the Vickers hardness of the heat-radiating plate isparticularly preferably from 19 to 30 kgf/mm².

[0057] The Vickers hardness is the hardness of the Al circuit or theheat-radiating plate, and different from the hardness of an Al platebefore the bonding. The Al plate is bonded to a ceramic substrate byusing the bonding material and by heating at a temperature of from 500to 640° C. Accordingly, the microstructure may change due to the heattreatment, and the bonding material may diffuse to decrease the Alpurity. Further, heat treatment may be carried out after the bonding,whereby the Al properties will change. Accordingly, it has littlesignificance to strictly define the hardness of the Al plate before thebonding.

[0058] The Vickers hardness is obtained by a method of pitching a fineindentater while applying a load thereto to measure the hardness, whichis widely used as a method for measuring the hardness of metals andceramics. Slightly different values may be obtained depending upon themeasuring conditions. Accordingly, in the present invention, themeasuring is carried out at a load of 1 kgf for a holding time of 15seconds.

[0059] Now, the ceramic substrate will be explained.

[0060] As the material for the ceramic substrate, aluminum nitride orsilicon nitride having a thermal conductivity of at least 70 W/mK issuitably used from the viewpoint that it is used for power modules whichrequire a high reliability. Silicon carbide, beryllium oxide or the likemay be used, however, they are poor in insulating properties and safety.

[0061] The ceramic substrate suitable for the present invention is analuminum nitride substrate having a thermal conductivity of at least 130W/mK and having such a X-ray diffraction peak intensity ratio that2≦Y₂O₃.Al₂O₃×100/AlN≦17 and 2Y₂O₃.Al₂O₃100/AlN≦2 on the surface by Cu—Kαray. By using said aluminum nitride substrate, the bonding strength bythe above Al—Cu type bonding material will increase, and a furtherhighly reliable circuit substrate will be obtained.

[0062] Such an aluminum nitride substrate can be formed by using (a) analuminum nitride powder material comprising from 1 to 10 wt % of coarseparticles having sizes of not smaller than 100 μm and from 10 to 50 wt %of fine particles having sizes of not larger than 1 μm, as measured bylaser diffraction scattering method, and by optimizing e.g. (b) thecomposition ratio of the Al₂O₃ content and the Y₂O₃ content in thealuminum nitride powder material, (c) the amount of oxygen increasedafter removal of a binder till before sintering and (d) the sinteringtemperature.

[0063] In a case where the 2Y₂O₃.Al₂O₃ content is high, for example, theAl₂O₃ content has to be relatively increased, and accordingly analuminum nitride powder material containing a large amount of oxygen isused, or Al₂O₃ is added, to decrease the Y₂O₃ content. On the otherhand, in a case where the Y₂O₃.Al₂O₃ content is high, the additionamount of Y₂O₃ is reduced, or the sintering temperature is lowered.Further, the Al₂O₃ content may be increased by carrying out removal of abinder in the air.

[0064] As the sintering aid, a powder of e.g. yttria, alumina, magnesiaor an oxide of a rare earth element, is incorporated in the aluminumnitride powder material in an amount of from about 0.5 to about 10 wt %based on the powder material. Forming is carried out by using an organicbinder such as butyral or methylcellulose, and after removal of thebinder, sintering is carried out by holding the formed product in anon-oxidizing atmosphere of e.g. nitrogen or argon at a temperature offrom 1700 to 1900° C. for from 1 to 12 hours.

[0065] The thickness of the ceramic substrate is usually 0.635 mm, butit can be changed according to the properties required. For example, ina case where heat resistance is important and insulating properties at ahigh voltage are not so important, a thin substrate of from 0.5 to 0.3mm may be used. On the other hand, in a case where insulating pressureresistances at a high voltage or partial discharge properties areimportant, a thick substrate of from 1 to 3 mm may be used.

[0066] Now, the process for producing the circuit substrate of thepresent invention will be explained.

[0067] The circuit substrate of the present invention is formed by e.g.a method of bonding an Al plate or an Al alloy plate to a ceramicsubstrate by using the above Al—Cu type bonding material under heating,followed by etching, or a method of bonding a circuit pattern punchedout of an Al plate or an Al alloy plate to a ceramic substrate by usingthe above Al—Cu type bonding material.

[0068] In either case, the bonding temperature is within a range of from540 to 640° C. However, the proper range is different depending upon thecomposition of the bonding material. In a case where the componenthaving a relatively low melting point such as Zn or In is incorporated,or the content of e.g. Cu or Mg is relatively high, the bonding will besatisfactorily carried out at a temperature of at most 600° C. On theother hand, if the bonding temperature exceeds 640° C., solderingdefects (moth-eaten phenomenon formed on the Al circuit) are likely toresult. Further, it is preferred to apply a pressure of from 1 to 100kgf/cm² in a direction perpendicular to the ceramic substrate during thebonding.

[0069] In conventional production of the circuit substrate, when themetal plate is bonded to the ceramic substrate, a pressure is appliedthereto by putting a weight thereon. However, the pressure is at mostabout 0.1 kgf/cm², and with a pressure at this level, the metal platewill follow only relatively mild warps and undulations. On the otherhand, in the present invention, a high pressure of from 1 to 100kgf/cm², which is extravagantly high in the prior art, is applied.Accordingly, a standard ceramic substrate will be used directly withoutrequiring strict smoothness and flatness, thus the productivity willimprove. Since the ceramic substrate has a high compressive strength, itis hardly damaged by the pressure at this level. However, as a ceramicsubstrate usually has a few warps and undulations, there is a risk ofbeing split if a pressure exceeding 50 kgf/cm², particularly a pressureexceeding 100 kgf/cm², is applied thereto, and thus the operation has tobe carried out cautiously.

[0070] Al or an Al alloy itself is a very soft metal at a temperature ofat least 500° C., as evident from the fact that it is annealed at atemperature of from 300 to 350° C. Accordingly, even if the solderingdefects are formed in the bonding, they will be crushed and disappear byapplying a pressure of from 1 to 100 kgf/cm² thereto. Accordingly, whenit is important to minimize the soldering defects, the bonded product isreheated at a temperature of at least 400° C. while applying pressurethereto after the bonding, or a pressure is applied thereto at atemperature of at least 400° C. in a cooling step after the bonding.

[0071] The pressure is applied in a direction perpendicular to theceramic substrate, and the method or the like is not particularlylimited. A method of putting a weight thereon, or a method ofmechanically sandwiching the substrate by using e.g. a jig, may, forexample, be employed.

[0072] The bonding material is an Al—Cu type alloy or a mixturecontaining Al and Cu, preferably an Al—Cu type alloy foil, morepreferably an Al—Cu—Mg type alloy foil comprising at least 86 wt % ofAl, from 1 to 6 wt % of Cu and at most 3 wt % (particularly from 0.2 to2.0 wt %) of Mg (not including 0). Further, a paste comprising a powderof this alloy or a metal powder mixture having this alloy compositionand an organic binder and a solvent, may be used. In this case, theoperation has to be carried out carefully so that the metal is notoxidized, and the amount of oxygen in the metal powder is adjusted to beat most 1 wt %, particularly at most 0.8 wt %. Further, in order tomaintain the above mentioned relation in thickness to the Al circuit,the thickness of the bonding material is calculated as the thickness ofthe alloy foil. Namely, a thickness of 100 μm of a paste layer having abulk density of 50% corresponds to a thickness of 50 μm of an alloyfoil.

[0073] The bonding material may be disposed either on the ceramicsubstrate or on the metal plate or the circuit pattern. Further, themetal plate or the circuit pattern may be preliminarily cladded in thealloy foil.

[0074] Then, the bonded product is etched, as the case requires. When apattern of a circuit or a heat-radiating plate is bonded, etching is notparticularly required. Etching may be carried out by a conventionalresist etching process. Further, a surface treatment such as plating maybe carried out, as the case requires.

[0075] The preferred conditions for the bonding in the present inventionare such that an Al—Cu type alloy foil having a thickness of from 15 to35 μm is used as the bonding material, the pressure applied is from 8 to50 kgf/cm², the bonding temperature is at least 590° C., and the holdingtime is at least 20 minutes under this pressure at this bondingtemperature, whereby the Vickers hardness of the Al circuit can easilybe made at most 16 kgf/cm². More preferably, the holding is carried outat a temperature of from 595 to 635° C. for from 20 to 90 minutes.

[0076] The mechanism how the Vickers hardness reduces in the presentinvention is not clearly understood in detail. It is considered thatsince the bonding in the present invention is carried out at anextravagantly high temperature as compared with the conventionalannealing temperature of from 300 to 350° C., Al is in a very softstate, whereby the pressure is likely to be transmitted uniformly, andthus the particle growth is suppressed.

[0077] Now, the present invention will be described in further detailwith reference to Examples. However, it should be understood that thepresent invention is by no means restricted to such specific Examples.

EXAMPLES 1 to 6 AND COMPARATIVE EXAMPLES 1 TO 3

[0078] As an aluminum nitride substrate, a sintered one was directlyused without post-processing such as surface polishing or curing ofwarps, and it had a thickness of 0.635 mm, a size of 2 inches square, athermal conductivity of 170 W/mK and a bending strength of 400 MPa.Further, as an Al plate for forming an Al circuit, JIS1090 (thickness:0.5 mm, Al purity: 99.9 wt %) was used.

[0079] Firstly, Al plates were overlaid on the front and the back of thealuminum nitride substrate by means of a bonding material, and theresulting assembly was sandwiched between C—C composite plates(thickness: 2 mm) and heated at a temperature of from 550 to 620° C.under vacuum or in N₂ while applying pressure thereto uniformly in adirection perpendicular to the ceramic substrate by a hot pressapparatus.

[0080] As the bonding material, one of (a) an Al-9.5wt % Si-1 wt % Mgalloy foil, (b) an Al-15 wt % Ge alloy foil, (c) an Al-4.1 wt % Cu-0.5wt % Mn alloy foil, (d) an Al-2.8 wt % Cu alloy foil, and (e) a pastecomprising a powder obtained by atomizing the above alloy foil (c) in N₂to the average particle size of 10 μm and an organic binder (PIBMA) anda solvent (terpineol), was used.

[0081] 100 bonded products were prepared in each Example, and bondingfailures and soldering defects were inspected under three timesmagnification by using soft X-rays. The lower limit of detection wasabout 0.3 mm in diameter. Further, 10 products were selected in eachExample, and etching was carried out by a FeCl₃ liquid within 2 mm fromthe periphery of either Al plate on each product, electroless Ni—Pplating was applied thereto in a thickness of 3 μm on either side, andthen two silicon chips of 12.5 mm square were soldered on the centerportion on the front side by an eutectic solder, and the opposite sidewas soldered to an Al/SiC heat sink. The soldering thickness was 150 μmon either side.

[0082] Then, heat history test for 3000 cycles and 5000 cycles, eachcycle consisting of 30 minutes at −40° C., 10 minutes at roomtemperature, 30 minutes at 125° C. and 10 minutes at room temperature,was carried out, and flaws in appearance such as blisters and peelingswere checked, and the presence or absence of solder cracks wereinspected by observing cross sections of three circuit substrates. Then,the circuit portion of each of seven circuit substrates was dissolved tomeasure the presence or absence of cracks by ink test method (redcheck). The results are shown in Table 1.

EXAMPLE 7 and COMPARATIVE EXAMPLE 4

[0083] Circuit substrates were produced in the same manner as in Example2 or Comparative Example 2, except that a silicon nitride substrate(thickness: 0.635 mm, size: 2 inches square, thermal conductivity: 70W/mK, bending strength: 800 MPa) was used instead of the aluminumnitride substrate. The obtained circuit substrates were evaluated in thesame manner as in Example 1. The results are shown in Table 1. TABLE 1Bonded product Bonding (number of material failures per 100 ThickBonding condition products) After 3000 cycles After 5000 cycles CeramicCompo- ness Pressure Bonding Soldering Solder Substrate Solder Substratesubstrate sition (μm) (kgf/cm²) Atmosphere failure defect crack crackcrack crack Ex. 1 AlN (c) 45 2 Vacuum 1/100 0/100 0/3 0/7 2/3 3/7 2 AlN(c) 25 10 In N₂ 0/100 0/100 0/3 0/7 1/3 1/7 3 AlN (c) 15 30 Vacuum 0/1000/100 0/3 0/7 1/3 1/7 4 AlN (d) 30 5 In N₂ 0/100 0/100 0/3 0/7 1/3 2/7 5AlN (d) 40 3 Vacuum 0/100 1/100 0/3 0/7 2/3 4/7 6 AlN (e) 20 20 Vacuum0/100 0/100 0/3 0/7 1/3 1/7 7 Si3N4 (c) 25 10 In N₂ 0/100 0/100 0/3 0/70/3 0/7 Comp. 1 AlN (a) 50 0.5 Vacuum 15/100  4/100 0/3 2/7 3/3 7/7 Ex.2 AlN (b) 30 2 Vacuum 0/100 6/100 2/3 0/7 3/3 7/7 3 AlN (b) 20 110Vacuum 2/100 8/100 0/3 3/7 3/3 7/7 4 Si3N4 (b) 30 2 Vacuum 1/100 7/1002/3 0/7 3/3 7/7

[0084] As evident from Table 1, in Examples 1 to 7 of the presentinvention, the bonding state was excellent, and circuit substrates werestably produced even in N₂, whereas in Comparative Examples 1 to 4, manyfailures occurred, and the productivity was poor. Further, surprisingly,properties in Examples of the present invention were equal to those inComparative Examples.

[0085] Further, the alloy foils (a) and (b) used in Comparative Exampleshave a composition as described in JP-A-3-125463, but they arecustom-made and not on the market as alloy foils, and thus they arehardly available. On the other hand, the alloy foil (c) used in Examplesis a commercial product made by forming 2017 alloy into a foil, and itis available easily at a low cost.

EXAMPLES 8 to 12

[0086] Al plates (purity: at least 99.99 wt %, thickness: shown in Table2) were overlaid on the front and the back of an aluminum nitridesubstrate (thickness: 0.635 mm, size: 2 inches square, thermalconductivity: 175 W/mK, three-point bending strength: 420 MPa) by meansof a bonding material, and a pressure was applied thereto in a directionperpendicular to the aluminum nitride substrate by using a jig whereincarbon plates were screwed and pressed on the substrate. Bondingconditions are shown in Table 2. As the bonding material, (f) anAl-3.9wt % Cu alloy foil or (g) a paste obtained in such a manner thatthe alloy foil (f) was atomized in N₂ to the average particle size of 9μm, the obtained particles having sizes of 45 μm or smaller werecollected, and an organic binder and a solvent were added thereto toobtain a paste, was used.

[0087] After the bonding, etching resists were printed by screenprinting, followed by etching by a FeCl₃ liquid. Patterns for an Alcircuit and a heat-radiating plate were squares (corner R: 2 mm), whichwere formed on the center portion of the ceramic substrate. Their sizeswere variously changed to adjust the volume ratio of the Al circuit tothe heat-radiating plate as shown in Table 2. Then, the resists weretaken off, and electroless Ni—P plating was applied in a thickness of 3μm on either side to obtain a circuit substrate.

[0088] The Vickers hardness of the obtained circuit substrate wasmeasured. Then, an Al wire of 300 μm was bonded thereto by supersonicwaves, and a Si chip of 13 mm square was soldered on the center portion.Then such samples were prepared, and subjected to heat cycle test in thesame manner as in Example 1. After the test, the presence or absence ofpeeling of the bonding wire or damages such as solder cracks, wasinspected, and then, the circuits and the heat-radiating plates weredissolved with hydrochloric acid to observe the presence or absence ofcracks on the aluminum nitride substrates. The results are shown inTable 3. TABLE 2 Thick- Volume ratio ness (Al circuit)/ of Al (heat-plate radiating Ex. Bonding material (μm) Bonding condition plate)  8(f) Thickness: 25 μm 300 605° C., 30 min., 1.1 in N₂, 20 kgf/cm²  9 (g)Thickness: 35 μm 500 620° C., 60 min., 0.9 in N₂, 10 kgf/cm² 10 (f)Thickness: 20 μm 200 600° C., 70 min., 0.8 in N₂, 40 kgf/cm² 11 (f)Thickness: 15 μm 150 610° C., 50 min., 1.2 in N₂, 50 kgf/cm² 12 (f)Thickness: 35 μm 400 595° C., 20 min., 1.3 in N₂,  3 kgf/cm²

[0089] TABLE 3 After After Vickers Peeling 3000 cycles 5000 cycleshardness of evaluation Sub- Sub- Al circuit Wire Solder strate Solderstrate Ex. (kgf/mm²) bonding Plating crack crack crack crack  8 13.7Peeling No 0/10 0/10 0/10 0/10 0/10 peeling  9 14.4 Peeling No 0/10 1/100/10 3/10 0/10 peeling 10 12.9 Peeling No 0/10 0/10 2/10 0/10 0/10peeling 11 13.2 Peeling No 0/10 0/10 2/10 0/10 0/10 peeling 12 15.0Peeling No 0/10 1/10 3/10 3/10 0/10 peeling

[0090] As evident from Tables 2 and 3, the bonding wire and the platingwere not damaged even after the heat cycle test for 5000 cycles, and thecracks on the solder and the aluminum nitride substrate could beminimized, by making the thickness of the Al circuit at least 100 μm andthe Vickers hardness at most 15 kgf/mm². Particularly, as evident fromthe comparison between Examples 8, 10 and 11 and Examples 9 and 12, theabove effects became significant when the thickness of the Al circuitwas from 150 to 300 μm, and the volume ratio of the Al circuit to theheat-radiating plate (circuit volume/heat-radiating plate volume) wasfrom 0.80 to 1.2.

EXAMPLES 13 to 18

[0091] Circuit substrates were produced by using the aluminum nitridesubstrate used in Example 1, in the same manner as in Example 1 exceptthat the Al plate (commercially available product), the bonding materialand the bonding conditions were as shown in Table 4, and evaluationswere carried out in the same manner as in Example 1. The results areshown in Table 5. TABLE 4 Bonding material Al plate Bonding conditionThickness Purity Thickness Pressure Ex. Composition (mm) (wt %) * (mm)(kgf/cm²) Temp., Time 13 93.8 wt % Al-5.0 wt % Cu-1.2 wt % Mg 0.02 99.850.4 10 600° C., 30 min. 14 94.5 wt % Al-4.0 wt % Cu-1.0 wt % Mg- 0.02599.90 0.4 20 615° C., 25 min. 0.5 wt % Mn 15 96.2 wt % Al-3.0 wt %Cu-0.8 wt % Mg 0.03 99.99 0.5 50 625° C., 20 min. 16 97.4 wt % Al-2.0 wt% Cu-0.6 wt % Mg 0.035 99.99 0.5 45 635° C., 60 min. 17 95.7 wt % Al-3.5wt % Cu-0.5 wt % Mg- 0.04 99.999 0.5 30 605° C., 15 min. 0.3 wt % Si 1894.1 wt % Al-5.5 wt % Cu-0.4 wt % Mg 0.015 99.85 0.3 12 595° C., 10 min.

[0092] TABLE 5 Bonding failure (number of After After failures 3000cycles 5000 cycles per 100 Solder Substrate Solder Substrate Ex.products crack crack crack crack 13 0/100 0/3 1/7 0/3 3/7 14 0/100 0/30/7 0/3 1/7 15 0/100 0/3 0/7 0/3 0/7 16 0/100 0/3 0/7 0/3 2/7 17 0/1000/3 0/7 0/3 1/7 18 0/100 1/3 1/7 2/3 4/7

[0093] As evident from the comparison between Table 1 and Tables 4 and5, by using, as the bonding material, one comprising at least 86 wt % ofAl, from 1 to 6 wt % of Cu and at most 3 wt % of Mg (not including 0),circuit substrates showing an excellent bonding state without e.g.voids, having substantially no solder cracks nor cracks on the aluminumnitride substrate even after the heat cycle test for 5000 cycles, andhaving a high durability against heat history, could be produced.

[0094] Further, as evident from the comparison between Examples 13 and18 and Examples 14 to 17, along with the increase in the Al platepurity, the number of failures due to substrate cracks tended todecrease even after the heat cycle test for 5000 cycles. Particularly,as evident from the comparison between Example 14 and Example 15, theabove effect became significant when a rolled Al plate having a purityof at least 99.99 wt % was used.

[0095] The reason why the best effect was obtained in Example 15 is thatthe thickness of the bonding material was optimized. From the comparisonbetween Example 16 and Example 17, in Example 17, the bonding materialwas thick, such being unfavorable to prevention of the substrate cracks,however, the Al plate purity was high, whereby the number of thesubstrate cracks was smaller than that of Example 16.

EXAMPLES 19 to 21

[0096] Production of Aluminum Nitride Substrate

[0097] Y₂O₃ was mixed with a commercially available aluminum nitridepowder in a proportion as shown in Table 6, an organic binder and anorganic solvent were added thereto, followed by kneading, and themixture was formed into a sheet by a roll forming machine. The sheet wascut, a releasing material (BN powder) was coated thereon, the cut sheetswere laminated one on another, the binder was removed at 450° C. under areduced pressure of about 1 Pa, and further, decarbonization was carriedout in the air. Each sample was baked in N₂ atmosphere under bakingconditions as shown in Table 6, to produce an aluminum nitride substrate4 having a size of 40 mm×40 mm and a thickness of 0.635 mm. With respectto the obtained aluminum nitride substrate, the intensity peak ratio onthe surface was measured by X-ray diffraction, and the thermalconductivity was obtained by laser flash method. The results are shownin Table 6.

[0098] Production of Circuit Substrate Composite

[0099] A heat sink 6 (commercially available Al/SiC composite having asize of 50 mm×50 mm and a thickness of 3 mm) and an Al circuit 2(circuit pattern stamped out from a commercially available Al material(purity: at least 99.99 wt %)) were laminated on either side of theobtained aluminum nitride substrate by means of a bonding material 3.5as shown in Table 7, as illustrated in FIG. 1. In FIG. 1, the numeral 1indicates a carbon spacer. The obtained laminate was heated in a furnacewhile applying pressure thereto in a direction perpendicular to thealuminum nitride substrate by means of a carbon push rod by using ahydraulic monoaxial pressurizing apparatus from outside of the furnace,for bonding. The bonding was carried out under vacuum of 4×10⁻³ Pa(batch type furnace) or in N₂ gas (continuous furnace) under conditionsas shown in Table 8.

[0100] With respect to the obtained circuit substrate composite, thebonding state was observed by a supersonic flaw detector (SAT), and onewherein non-bonded portion having a diameter of 1 mm or larger or anon-bonded portion area of at least 1% was found, was rated as bondingfailure. Then, each sample was subjected to heat cycle test for 3000cycles and 5000 cycles, each cycle comprises 30 minutes at −40° C., 10minutes at room temperature, 30 minutes at 125° C. and 10 minutes atroom temperature, whereupon the appearance was observed to confirm thepresence or absence of cracks, and then the bonding state was checkedagain by SAT. The results are shown in Table 9. TABLE 6 Y₂O₃ additionX-ray Thermal amount Baking condition diffraction* conductivity Ex. (wt%) Temp. (° C.) Time (hrs) X Y Z (W/mk) 19 4.7 1775 12.5 0 5 1 187 203.5 1800 8 0 6 0 176 21 2.8 1815 2.5 1 9 0 164

[0101] TABLE 7 Bonding material Thickness of Thickness Al circuit Ex.Composition (mm) (mm) 19 95.0 wt % Al-4.0 wt % Cu-1.0 wt % Mg 0.03  0.420 93.5 wt % Al-5.0 wt % Cu-1.5 wt % Mg 0.015 0.5 21 95.3 wt % Al-3.5 wt% Cu-1.2 wt % Mg 0.025 0.4

[0102] TABLE 8 Bonding condition Temp. Time Pressure Ex. Atmosphere (°C.) (min) (kgf/cm²) 19 N₂ 615 10 35 20 Vacuum 600 20 45 21 N₂ 620  3 15

[0103] TABLE 9 Initial number of Number of failures Number of failuresfailures after 3000 cycles after 5000 cycles Ex. Appearance SATAppearance SAT Appearance SAT 19 No flaw 0/10 No flaw 0/10 No flaw 0/1020 No flaw 0/10 No flaw 0/10 No flaw 1/10 21 No flaw 0/10 Solder crack:1/10 Solder crack: 3/10 1/10 5/10

[0104] As evident from Tables 6 to 9, by using the aluminum nitridesubstrate having a thermal conductivity of at least 130 W/mK and such aX-ray diffraction peak intensity ratio on the substrate surface that2≦Y₂O₃.Al₂O₃×100/AlN≦17 and 2Y₂O₃.Al₂O₃×100/AlN≦2, circuit substratecomposites showing an excellent bonding state and having few cracks evenafter the heat cycle test for 5000 cycles, were obtained. Particularly,in Examples 19 and 21, circuit substrate composites having few bondingfailures could be produced even by using the simplified continuousfurnace.

EXAMPLES 22 to 25

[0105] An Al plate for an Al circuit and an Al plate for aheat-radiating plate were overlaid respectively on the front and on theback of the aluminum nitride substrate used in Example 8 by means of abonding material as shown in Table 10, and bonding was carried out underbonding conditions as shown in Table 11.

[0106] After the bonding, etching resists were printed by screenprinting, followed by etching by a FeCl₃ liquid. Patterns for an Alcircuit and a heat-radiating plate were squares (corner R: 2 mm), whichwere formed on the center portion of the ceramic substrate. Their sizeswere variously changed to adjust the volume ratio of the Al circuit tothe heat-radiating plate as shown in Table 10. Then, the resists werepeeled, and electroless Ni—P plating was applied in a thickness of 3 μmon either side to obtain a circuit substrate.

[0107] By using some of the obtained circuit substrates, the Al circuitand the heat-radiating plate were peeled to measure their Vickershardnesses. Further, a Si chip of 13 mm square was soldered on thecenter portion of each of some circuit substrates, and heat cycle testwas carried out in the same manner as in Example 19. The results areshown in Table 12. TABLE 10 Al plate for Al Al plate for heat- Bondingmaterial circuit radiating plate Thick- Thick- Thick- Volume ratio (Alness ness ness circuit)/(heat- Ex. Composition (mm) (mm) Type* (mm)Type* radiating plate) 22 95.0 wt % Al-4.0 wt % Cu-1.0 wt % Mg 0.02 0.4At least 0.4 At least 0.90 99.99 wt % 99.3 wt % 23 94.2 wt % Al-4.5 wt %Cu-0.8 wt % Mg- 0.02 0.5 At least 0.4 At least 1.05 0.5 wt % Mn 99.99 wt% 99.5 wt % 24 93.5 wt % Al-5.0 wt % Cu-l.5 wt % Mg 0.015 0.5 At least0.5 Al-0.5 0.88 99.9 wt % wt % Si alloy 25 95.3 wt % Al-3.5 wt % Cu-l.2wt % Mg 0.025 0.4 At least 0.5 Al-0.3 1.12 99.85 wt % wt % Mg alloy

[0108] TABLE 11 Bonding condition Vickers hardness (kgf/mm²) Bonding AlHeat-radiating Ex. Temp. Time Atmosphere circuit plate 22 625° C. 10 minN₂ 14.8 23.9 23 615° C. 15 min Vacuum 13.7 22.3 24 595° C.  5 min Vacuum15.4 27.2 25 635° C. 20 min Vacuum 16.3 28.7

[0109] TABLE 12 Number of failures Number of failures after 3000 cyclesafter 5000 cycles Solder Solder under Solder Solder under Sub- underchip substrate Sub- under chip substrate Ex. strate (SAT) (SAT) strate(SAT) (SAT) 22 0/10 0/10 0/10 0/10 0/10 0/10 23 0/10 0/10 0/10 1/10 0/101/10 24 1/10 0/10 0/10 3/10 3/10 3/10 25 0/10 0/10 1/10 0/10 5/10 5/10

[0110] As evident from the comparison between Examples 22 to 24 andExample 25, with respect to the circuit substrates having a volume ratioof the Al circuit to the heat-radiating plate of from 0.80 to 1.2, bymaking the Vickers hardness of the Al circuit at most 16 kgf/mm² and theVickers hardness of the heat-radiating plate from 19 to 30 kgf/mm², thesolder cracks and substrate cracks could be significantly suppressedeven after 5000 cycles.

[0111] Here, in Examples 13 to 25, the Cu content in the layercomprising Al and Cu in each bonded layer was confirmed to be from 1 to6 wt %, by quantification of the peak ratio in elemental analysis bycross section observation.

[0112] According to the present invention, a highly reliable circuitsubstrate useful for power modules can be provided stably at a low cost.

[0113] Further, according to the present invention, a highly reliablecircuit substrate with few solder cracks and cracks on a ceramicsubstrate, can be provided.

[0114] Still further, according to the present invention, a highlyreliable circuit substrate composite (circuit substrate with heat sink)having a light weight and excellent heat-radiating properties, can beprovided.

[0115] The entire disclosure of Japanese Patent Application No.11-149302 filed on May 28, 1999 including specification, claims,drawings and summary are incorporated herein by reference in itsentirety.

What is claimed is:
 1. A circuit substrate which has a ceramic substrateand an Al circuit comprising Al or an Al alloy bonded to said ceramicsubstrate via a layer comprising Al and Cu.
 2. The circuit substrateaccording to claim 1, wherein the ceramic substrate is an aluminumnitride substrate or a silicon nitride substrate.
 3. The circuitsubstrate according to claim 1, wherein the ceramic substrate is analuminum nitride substrate having a thermal conductivity of at least 130W/mK and having such a X-ray diffraction peak intensity ratio that2≦Y₂O₃.Al₂O₃×100/AlN≦17 and 2Y₂O₃.Al₂O₃×100/AlN≦2 on the surface.
 4. Thecircuit substrate according to claim 1, wherein the Al circuitcomprising Al or an Al alloy is formed by using Al having a purity of atleast 99.85 wt %.
 5. The circuit substrate according to claim 1, whereinthe Al circuit comprising Al or an Al alloy is formed by using a rolledAl having a purity of at least 99.99 wt %.
 6. The circuit substrateaccording to claim 1, wherein the proportion of Cu in the layercomprising Al and Cu is from 1 to 6 wt %.
 7. The circuit substrateaccording to claim 1, wherein the Al circuit comprising Al or an Alalloy has a thickness of at least 100 μm and a Vickers hardness of atmost 15 kgf /mm².
 8. The circuit substrate according to claim 1, whichhas a heat-radiating plate comprising Al or an Al alloy formed on theceramic substrate on the side (back side) opposite to the side on whichthe Al circuit is formed.
 9. The circuit substrate according to claim 8,wherein the volume ratio of the Al circuit to the heat-radiating plateis from 0.80 to 1.2.
 10. The circuit substrate according to claim 8,wherein the Al circuit has a Vickers hardness of at most 16 kgf /mm²,and the heat-radiating plate has a Vickers hardness of from 19 to 30kgf/mm².
 11. A circuit substrate which has a ceramic substrate and an Alcircuit comprising Al or an Al alloy bonded to said ceramic substrate byusing, as a bonding material, an Al—Cu type alloy or a mixturecontaining Al and Cu.
 12. The circuit substrate according to claim 11,wherein the bonding material is an Al—Cu type alloy foil.
 13. Thecircuit substrate according to claim 11, wherein the ceramic substrateis an aluminum nitride substrate or a silicon nitride substrate.
 14. Thecircuit substrate according to claim 11, wherein the ceramic substrateis an aluminum nitride substrate having a thermal conductivity of atleast 130 W/mK and having such a X-ray diffraction peak intensity ratiothat 2≦Y₂O₃.Al₂O₃×100/AlN≦17 and 2Y₂O₃.Al₂O₃×100/AlN≦2 on the surface.15. The circuit substrate according to claim 11, wherein the Al circuitcomprising Al or an Al alloy is formed by using Al having a purity of atleast 99.85 wt %.
 16. The circuit substrate according to claim 11,wherein the Al circuit comprising Al or an Al alloy is formed by using arolled Al having a purity of at least 99.99 wt %.
 17. The circuitsubstrate according to claim 11, wherein the bonding material comprisesat least 86 wt % of Al, from 1 to 6 wt % of Cu and at most 3 wt % of Mg(not including 0).
 18. The circuit substrate according to claim 11,wherein the Al circuit comprising Al or an Al alloy has a thickness ofat least 100 μm and a Vickers hardness of at most 15 kgf/mm².
 19. Thecircuit substrate according to claim 11, which has a heat-radiatingplate comprising Al or an Al alloy formed on the ceramic substrate onthe side (back side) opposite to the side on which the Al circuit isformed.
 20. The circuit substrate according to claim 19, wherein thevolume ratio of the Al circuit to the heat-radiating plate is from 0.80to 1.2.
 21. The circuit substrate according to claim 19, wherein the Alcircuit has a Vickers hardness of at most 16 kgf/mm², and theheat-radiating plate has a Vickers hardness of from 19 to 30 kgf/mm².22. A process for producing a circuit substrate, which comprisesdisposing an Al or Al alloy plate, pattern or both on a ceramicsubstrate by means of, as a bonding material, an Al—Cu type alloy or amixture containing Al and Cu, and heating the resulting assembly at atemperature of from 540 to 640° C. while applying a pressure of from 1to 100 kgf/cm² thereto in a direction perpendicular to the ceramicsubstrate to bond the Al or Al alloy plate, pattern or both to theceramic substrate, followed by etching as the case requires.
 23. Theprocess for producing a circuit substrate according to claim 22, whereinthe bonding material is an Al—Cu type alloy foil.
 24. The process forproducing a circuit substrate according to claim 23, wherein the bondingmaterial is an Al—Cu type alloy foil having a thickness of from 15 to 35μm, the Al or Al alloy plate, pattern or both, having a thickness of atleast 100 μm, is disposed on either side of the ceramic substrate bymeans of said bonding material, and the resulting assembly is held underheating at a temperature of at least 590° C. for at least 20 minuteswhile applying a pressure of from 8 to 50 kgf/cm² thereto in a directionperpendicular to the ceramic substrate.
 25. The process for producing acircuit substrate according to claim 23, wherein the Al—Cu type alloyfoil is an Al—Cu—Mg type alloy foil comprising at least 86 wt % of Al,from 1 to 6 wt % of Cu and at most 3 wt % of Mg (not including 0). 26.The process for producing a circuit substrate according to claim 24,wherein the Al—Cu type alloy foil is an Al—Cu—Mg type alloy foilcomprising at least 86 wt % of Al, from 1 to 6 wt % of Cu and at most 3wt % of Mg (not including 0).